Photoconductors for radiation imaging detectors: materials issues and device designs

在各种医学放射成像方式中，特别是在普通x射线摄影和实时辐射成像中，对降低照射剂量的需求很大。经过近二十年的广泛研究，基于非晶硒（a-Se）的直接转换平板数字x射线探测器（入射x射线直接在光导体层产生载流子）最近被商业化用于数字乳房x光检查。a-Se探测器并不完美，常规a-Se探测器在正常工作下的主要缺点是与其他潜在的光导体（如多晶氧化铅或碘化汞）相比灵敏度低。低灵敏度导致低剂量成像的信噪比低，严重影响图像的诊断特征。极低剂量医用x射线成像可以通过以下方式实现：(1)利用a- se层在非常高的电场下的雪崩倍增过程获得更高的电荷信号，和/或(2)用其他可以提供更高收集电荷的潜在光导体取代a- se。在x射线探测器中利用有机光导体可以获得低成本的x射线成像。雪崩a- se固体成像探测器的研究还处于非常不成熟的阶段；它需要广泛的研究工作，以清楚地了解载流子产生，倍增和传输机制的基本潜在物理，并通过检查成像性能来优化探测器设计。PI的研究计划将努力开发低剂量和/或低成本的医用x射线/光学成像探测器，通过广泛研究光电导体中的电荷载流子产生、传输、倍增和噪声产生机制，开发用于分析成像探测器性能的数学模型，如暗电流、灵敏度、DQE（探测量子效率）和MTF（调制传递函数）作为场、温度、探测器结构的函数。x射线诱导效应，过量噪声，从而优化探测器设计。所提出的工作对于了解x射线/光学探测器操作和光电导体特性的基本物理特性，以及确定限制探测器性能的重要因素至关重要，这最终可以找到用于各种数字医学成像应用的低成本和高效辐射成像探测器的高效材料和设计。本研究将促进对非晶和多晶光导体高场输运机制的科学认识。数学模型可以被学术界和工业界的研究人员用作设计工具。提议的工作为研究生的原创和基础研究提供了极好的范围，这将推进基础科学知识并将其传递给下一代。这项研究将有利于加拿大在医疗数字成像和数字辐射图像探测器社区（例如，Analogic和Hologic）的研发工作。

There is a huge demand for lowering irradiation dose in various medical radiation imaging modalities especially in general X-ray radiography and real-time radiation imaging. After last two decades of extensive research, amorphous selenium (a-Se) based direct conversion flat-panel digital X-ray detector (the incident X-rays directly generate charge carriers in the photoconductor layer) is recently commercialized for digital mammography. The a-Se detector is not perfect and the main drawback of the conventional a-Se detector under normal operation is its low sensitivity compared to other potential photoconductors such as polycrystalline lead oxide or mercuric iodide. Low sensitivity gives low signal to noise ratio in low-dose imaging and thus severely affects the diagnostic features of the image. Very low-dose medical X-ray imaging could be achieved by; (1) utilizing avalanche multiplication process at a very high electric field in a-Se layer for higher charge signal, and/or (2) replacing a-Se by other potential photoconductors that can provide higher collected charge. Low-cost X-ray imaging can be obtained by utilizing organic photoconductors in X-ray detectors. The research on avalanche a-Se solid state imaging detector is in a very premature level; it needs extensive research works to clearly understand the fundamental underlying physics of carrier generation, multiplication, and transport mechanisms, and to optimize the detector design by examining imaging performances. The PI's research program will endeavor to develop low-dose and/or low-cost medical X-ray/optical imaging detectors by extensively investigating charge carrier generation, transport, multiplication, and noise creation mechanisms in photoconductors, developing mathematical models for analyzing imaging detector performances such as dark current, sensitivity, DQE (detective quantum efficiency), and MTF (modulation transfer function) as a function of field, temperature, detector structure, X-ray induced effects, excess noise, and hence optimizing the detector designs. The proposed work is vital to understand the fundamental physics of X-ray/optical detector operations and photoconductor properties, and to identify the important factors that limit the detector performances, which can ultimately lead to finding the efficient materials and designs for low-cost and efficient radiation imaging detectors for various digital medical imaging applications. This research will advance scientific knowledge in high field transport mechanisms in amorphous and polycrystalline photoconductors. The mathematical models can be used as design tools by researchers in academia and industries. The proposed work has excellent scope for original and fundamental research for graduate students, which will advance fundamental scientific knowledge and transfer it to the next generation. This research will be beneficial to Canada’s R&D efforts in medical digital imaging, and the digital radiation image detector community (e.g., Analogic and Hologic).